A switched-capacitor converter (SCC) is a type of direct-current (DC) to DC voltage converter that achieves high efficiency and high power density by using switches and capacitors to transfer power from an input power supply to an output. An SCC includes a rectifier at its output, and a series of power switches interposed between the input power supply and the rectifier. Such an SCC further includes one or more legs, each of which includes at least a capacitor that couples one of the switches to the rectifier. The switches are controlled such that charge is transferred to and from the capacitor within each leg so that, for a step-down SCC, the voltage of the input power supply is down-converted to provide a reduced voltage at the output of the SCC. Some of the legs may also include inductors connected in series with the capacitors so as to provide a resonance that enables efficient soft-switching of the switches.
At the initiation of a start-up (power-up) of an SCC, the voltages across the capacitors within the SCC are typically zero, i.e., the capacitors are in a discharged state prior to application of an input power supply to the SCC. During steady-state operation of the SCC, however, each capacitor maintains an average voltage across it, together with some ripple associated with the energy transfers within the SCC. The discharged capacitors can cause a problem in that current flow through the SCC may spike to very high levels, sometimes termed “inrush current,” upon application of the input power supply to the SCC as the capacitors are charged at start-up to the average voltages necessary for steady-state operation of the SCC. This high start-up current causes electrical stress that can damage the SCC components, including the switches, capacitors and inductors. Additionally, the high start-up current can cause problematic dips in the voltage of the input power supply and/or lead to unwanted triggering of overcurrent protection circuits. The inrush current should be limited to prevent these problems.
One solution for limiting the current at the start-up of the SCC is to ramp up the voltage that is provided to the SCC input by the input power supply. For example, a current-limiting circuit may be placed between the input power supply and the SCC input, wherein the current limiting circuit includes a shunt resistor placed in parallel with a power switch. During start-up, the shunt resistor limits the current flowing into the SCC. Upon reaching steady-state operation of the SCC, the power switch is turned on to bypass the shunt resistor. While such a solution addresses the inrush current problem, it comes at the expense of adding an additional power switch to the SCC. In addition to the cost and size of the power switch, requiring current to flow through another power switch during steady-state operation reduces the efficiency of the SCC.
Another solution is to pre-charge the capacitors within an SCC to their desired steady-stage average voltages during the start-up phase of the SCC. Such pre-charging may involve additional pre-charging circuitry and/or special control of the switches within the SCC during the start-up phase. More particularly, this pre-charging often requires that voltages across the SCC capacitors be sensed so that the switches may be appropriately controlled to ensure each capacitor is charged to its desired (steady-state) voltage. While pre-charging circuitry and control may be feasible for an SCC that is monolithically integrated, such pre-charging becomes considerably more complex for high-power SCCs that typically require discrete power switches. In any case, capacitor pre-charging requires additional circuit and control complexity, and often requires additional current limiting as described in the above solution, all of which is undesirable.
Circuits and associated techniques are desired for limiting inrush current while requiring minimal additional circuitry, requiring low complexity, and minimizing power losses.